In the packaging industry, particularly in food processing, it is common to package perishables in sealed containers formed of, for example, foil, paper, metal, plastic, ceramic or combinations thereof. The contents may be deposited in an internal material receiving compartment formed in a body or receptacle portion of the container and then covered with a flexible cover that is glued or heat sealed about the compartment. With bags or packet type containers, the contents are often inserted through an open end of the container which is subsequently sealed. In order to prevent contamination and subsequent spoiling of perishables packed in such containers, it is essential that the seal be airtight.
During packaging some of the containers develop leaks or otherwise are not sealed with an airtight seal. It is particularly important in the food packaging industry to detect such defectively sealed containers before the containers are shipped to retail outlets and subsequently purchased by a consumer. It is also important to detect defective container seals early in the processing or packaging cycle to minimize the cost associated with further processing and handling of such containers and their contents. This is an especially difficult problem with bags or packaging where portions of the lid or cover and the container body such as a bowl, tray or other surface opposite the cover, extend outwardly beyond the seal. The outward projections obstruct the seal which would otherwise be available for test procedures as disclosed in U.S. Pat. No. 4,747,298. Product quality control considerations often require testing of every container after it has been sealed. Because of this, it is not reasonable to destructively test the containers, so a reliable non-destructive testing system must be used.
Container leak testing apparatus used to test each container should be capable of fast operation in order to handle the high throughput of many high speed packaging and container sealing processes. It is further desirable to have a container testing system which can be integrated into such high speed processing lines and used to provide reliable proof testing of the containers for leaks.
There have been numerous prior art attempts to develop reliable high speed, non-destructive container seal testing equipment. For example, U.S. Pat. No. 4,409,818 to Wyslotsky discloses a seal testing device and method for testing the integrity of a seal in a sealed package. The package is required to include a continuous seal adjacent the sealed material holding compartment. The seal is formed between two films that form the package and that extend outward of the seal to define a marginal edge portion of the package. A clamp arrangement of the device is used to bind the marginal edge portion outward of the seal. An air supply is used to blow relatively low pressure air between the two films. The air is supplied through a centrally located preformed hole in the upper cover film. A detector is used to detect deflection of the package responsive to a leak if present in the seal. The device requires that a hole be present in the film for such testing purposes and, further requires use of a combination vacuum-pressure head which engages outer surfaces of the lid to blow air down through the hole in the lid. The vacuum arrangement is used to draw one film away from the other to permit supply of air through the hole. The fluid supply includes a tube that is centered within the vacuum arrangement and which must be aligned with the hole in order to deliver fluid pressure to the area between the films separated by the vacuum arrangement. The vacuum force required to separate the films could have the effect of separating the seal and thereby producing a leak where none previously existed. Furthermore, the alignment of the vacuum head, the supply head, with the hole in the adjacent film must be precise because of the dual pressure-vacuum functions else the desired separation of the films will not take place or the fluid injection will not properly occur, such as where the vacuum head overlaps the hole.
U.S. Pat. No. 4,774,830 to Hulsman discloses a device which was intended to overcome problems perceived in the Wyslotsky device described above. The Hulsman apparatus makes use of an externally applied pressure chamber for isolating the external edge of a flange seal and applying a test pressure thereto. Containment of the pressure chamber includes sealing pressure applied directly to the seal which is being tested for leaks. A self compensating biasing mechanism is provided in an attempt to minimize the clamping force which necessarily occurs against the seal. The clamping device of the Hulsman apparatus may defeat the purpose of proof testing by temporarily sealing small leaks which exist in the container seal. Such leaks occur in seals that have small entrapped air or moisture pockets or other defects. Such defects may be flattened and sealed by the clamping pressure applied to the container seal which is required to maintain the external pressure chamber in the Hulsman device. Such defective containers will pass the Hulsman proof test but may later re-open. Such testing thus suffers a serious reliability problem which is addressed by the current invention.
Other conventional apparatus utilized for placing contents of a package under "external" vacuum to determine leaks represent earnest attempts at solutions to the problem of testing closed containers yet most are relatively slow and dependent upon unpredictable variables such as packaging material variances and compressibility of package contents and the tendency for the package contents to block leaks.
The advantage of vacuum test systems using internal vacuum forces within the packaging is that leaks, if present, will occur from the atmosphere inwardly toward the package contents. There is no opportunity for the package contents to block the leak and thereby provide false indication that a leak is not present. To the contrary, inward motion of air or other gas through a leak will have a tendency to unplug a leak and thus provide a relatively reliable indication of the leak, if accurate detection mechanisms are utilized. However, such procedures and apparatus have not been sufficiently reliable for use in packaging where portions of the cover "flap" and the container substrate "lip" extend outwardly beyond the seal being tested because of the tendency for the vacuum pressure to pull the cover flap tightly against the substrate lip. This action could result in a dangerous situation in which leaks would be temporarily closed during the testing procedure, but that would reopen at a later time.
Container seal testing problems are amplified in areas where multiple containers are formed and covered with a single cover. Such multiple container construction often occurs just prior to filling of the containers with a food or other product. Then, immediately following the filling procedure, the single continuous cover is applied over the open container ends. Testing of these "form and fill" type containers is a difficult and time consuming undertaking. Procedures involving destructive testing may be utilized on the individual containers. However, destructive type testing results in loss of product. Further, it is typically not feasible to recycle product from a destructively tested container if the container is interconnected integrally with a number of other assumed satisfactory containers or filled containers. Thus, adequate containers and testing apparatus, especially in compound container and cover systems has remained a difficult problem in the container forming and food packaging industries.
From the above, it is readily seen that attempts have been made to test for leaks in sealed containers. While the solutions offered and suggested by the above references in many cases function adequately for specialized applications, there remains a need for testable containers and for a fast and more reliable testing method and apparatus by which container seals, such as those that are overlapped outwardly by container members, may be tested for leaks. There also remains a strong need to proof test without applying any clamping forces to the seal being tested.